Various means have been used in the prior art to evaluate electronic circuitry signal processing performance. One measurement that was used in the past that is indicative of the fidelity of performance of the signal processing circitry, that is where the output is intended to faithfully resemble the input, is the phase relationship of the output with respect to the input.
A prior art diagnostic technique included the visual evaluation of wave forms by observing input and output signals on a dual channel oscilloscope. Although this prior art technique was effective in frequently caused problems because the instrumentation was expensive to purchase, complex to operate, difficult to interpret, and rarely easily portable. This technique requires knowledge of both the instrumentation and the nature of the observed waveforms. For example in an acoustic environment all frequencies of the signal are delayed by the propagation time from loudspeaker to microphone. A delay of approximately one foot per millisecond in propagation time represents a substantial error imparted to the aforementioned phase measurement test device. Another problem with checking phase in acoustic environments is the fact that loudspeakers are generally nonlinear in frequency response and therefore phase. Thus, the harmonics of a test signal will usually undergo a phase shift with respect to a test signal fundamental frequency, distorting the visual integrity of the test waveform. In order to overcome this problem as operator frequently must test at multiple frequencies and then either integrate by eye or average the curves using his best judgement. Similar problems are encountered in other circuitry not having a flat frequency response characteristics, such as transmission lines and transmission media.
Another method used in the prior art to evaluate circuit performance was to sum the input and output waveforms from a system under test. A large peak-to-peak voltage would indicate an in-phase condition. A low or zero peak-to-peak voltage would indicate an out-of-phase condition. The first problem with this technique is that it only works if the input and output amplitudes of the signal are normalized or made equal peak-to-peak. A second requirement is that the propagation delay through the system under test be negligible compared to the test signal wavelength. Thirdly, it must be practical to connect both the input and the output of the system under test to the analysis tool.